In the production of wood gas particularly from wood material, and even more specifically from chips, several options are available.
From wood material, such as chips, can be produced wood gas by introducing excess air into a fuel and air mixing chamber or mixer head. In this case, however, the resulting gas contains a fairly large amount of unburned inert nitrogen gases and the combustion value of generated wood gas per unit of supplied wood material remains low.
In case an attempt is made to increase the amount of gas per unit of supplied wood material by introducing pure oxygen into the mixer head, the production costs will rise dramatically because of the price of oxygen feed.
Both foregoing problems are at least partially avertable in so-called pyrolysis, wherein wood material, such as chips, is heated while attempting at the same time to avoid the passage of air or oxygen into the mixer head or gasification space. Although the pyrolysis of wood material, such as chips, provides a solution to the foregoing problems relating to the produced gas having a low combustion value per unit of supplied wood material, one major problem in pyrolysis results from impurities which migrate from wood material to gas fraction and hamper further processing of the product for example with a well-known Fischer-Tropsch process, and from impurities, most notably tars, which are left in so-called pyrolysis oil as a result of incomplete combustion. The elimination of impurities contained in pyrolysis oil and in gas fraction entails normally a considerable increase in pyrolysis costs.
The pyrolysis-generated impurities can be reduced with a so-called ablative pyrolysis in which wood material, such as chips, is gasified at a high temperature and at a relatively high pressure, but the problem here turns out to be a low conversion of wood material, resulting in 30-35% of unburned carbon residue per kilogram of supplied dry wood material.